A variety of methods for producing electrically conductive zinc oxide are known. For instance, Japanese Patent Laid Open Corresponding to OLS in Germany (hereunder referred to as "J.P. KOHKAI") Nos. Sho 58-161923, Sho 58-145620, Sho 55-162477, Sho 59-97531, Japanese Patent Publication corresponding to Auslege in Germany (hereinafter referred to as "J.P. KOHKOKU") No. Sho 55-19897 and U.S. Pat. No. 3,538,022 disclose methods which comprise adding to zinc oxide powder an oxide of at least one metal selected from the group consisting of aluminum, gallium, indium, tin and the like as a dopant to admix them and then heating the mixture at a temperature ranging from 600.degree. C. to 1200.degree. C. In addition, there are also dry methods which are known such as those comprising heating a mixture of zinc oxide powder and a dopant in the presence of solid carbon to calcine the mixture; wet methods such as those disclosed in J.P. KOHKAI Nos. Sho 62-35970, Sho 61-86421 and Sho 61-40338, which comprise neutralizing a mixture of a solution of a water-soluble zinc compound and a solution of a water-soluble compound of the aforesaid metals with an aqueous solution of an alkali hydroxide or an alkali carbonate or the like to form co-precipitates and heating and calcining the co-precipitates at a temperature of 500.degree. C. to 1000.degree. C. in nitrogen atmosphere or in a reducing atmosphere after washing, drying or preheating the coprecipitates; and the like.
However, in order to impart sufficient electric conductivity to a calcined product a dry method is necessary to perform calcination at an elevated temperature under a reducing atmosphere. This is accompanied by volatilization of zinc oxide due to reduction. Consequently, the yield of the desired product is lowered remarkably. Moreover, there is sometimes observed secondary sintering of the products during the high temperature calcining step.
On the other hand, in all of the conventional wet methods for producing electrically conductive zinc oxide, neutralization is performed in a batchwise operation. More specifically, an aqueous solution of an alkali hydroxide or an alkali carbonate is gradually added to an aqueous solution of a water-soluble compound of zinc and a water-soluble compound of a metal as a dopant so that a final pH of the reaction system falls within the range within which zinc and the like cause precipitation. In such a method, pH at the beginning of the neutralization differs from that at the final stage. Therefore, it is difficult to coprecipitate two or more of products having different rates of hydrolysis or rates of dissolution in a desired ratio so as to achieve a homogeneous distribution of the production in the solid phase. Accordingly, to impart a desired degree of electric conductivity to the resulting product, calcination is inevitably carried out under a reducing condition at an elevated temperature. However, as set out above, such a calcination at an elevated temperature is accompanied by lowering of product yield as a result of volatilization of zinc oxide. Furthermore, the reducing atmosphere in which the calcination is carried out exerts influence on the product to thus cause partial reduction of the resulting metal oxide, which in turn is accompanied by deposition of metals in a trace amount. This causes the lowering in whiteness of the resultant calcined product.
Accordingly, an object of the present invention is to provide a method for producing calcined products having a desired electrical conductivity with low temperature calcination.